At least six suspected militants were killed in a US drone strike early Wednesday in the Miramshah tehsil of North Waziristan tribal region where a comprehensive military operation launched by the Pakistani army against foreign and local terrorists is also under way.

Intelligence sources said six missiles fired by a drone hit a compound and a vehicle in Dargah Mandi village in Miramshah.

The identities of those killed in the strikes have not yet been ascertained, sources said.

Wednesday's is the third drone attack in a month, the earlier two successive US drone strikes killing at least 16 suspected militants in North Waziristan.

The army launched its long-awaited major operation 'Zarb-i-Azb' in the tribal region a week after an attack on the airport in Karachi, deploying troops, tanks and jets to the area in the crackdown on the Taliban and other militants.

A total of at least 187 suspected militants and eight soldiers have been reported killed since Saturday night when the operation was launched.

During the operation, the army has reportedly so far resorted only to air strikes, sending F-16 and Mirage fighter jets to pound suspected militant hideouts up in the mountains.

The ground offensive is expected to start in the next week when land forces will try to comb through North Waziristan's valleys and take over villages and buildings.

Air raids have continued daily since Sunday killing scores of militants, according to Pakistani military sources.